Aminolevulinic acid-mediated photodynamic therapy (ALA-PDT) is an emerging treatment for cancers. ALA, given as a
prodrug, selectively accumulates and is metabolized in cancer cells to form protoporphyrin IX (PpIX). Targeted local
irradiation with light induces cell death. Since the efficacy of ALA-PDT for large or deep tumors is currently limited, we
are developing a new approach that combines differentiation-inducing agents with ALA-PDT to improve the clinical
response. Here, we tested this new combination paradigm in the following two models of skin carcinoma in mice: 1)
tumors generated by topical application of chemical carcinogens (DMBA-TPA); 2) human SCC cells (A431) implanted
subcutaneously. To achieve a differentiated state of the tumors, pretreatment with a low concentration of methotrexate
(MTX) or Vitamin D (Vit D) was administered for 72 h prior to exposure to ALA. Confocal images of histological
sections were captured and digitally analyzed to determine relative PpIX levels. PpIX in the tumors was also monitored
by real-time in vivo fluorescence dosimetry. In both models, a significant increase in levels of PpIX was observed
following pretreatment with MTX or Vit D, as compared to no-pretreatment controls. This enhancing effect was
observed at very low, non-cytotoxic concentrations, and was highly specific to cancer cells as compared to normal cells.
These results suggest that use of differentiating agents such as MTX or Vit D, as a short-term combination therapy given
prior to ALA-PDT, can increase the production of PpIX photosensitizer and enhance the therapeutic response of skin
cancers.